Because liquid crystal display devices are display devices that enable weight reduction, thinning, and lower power consumption, liquid crystal display devices are widely utilized for televisions, monitors for personal computers, monitors for portable terminals and the like. In such liquid crystal display devices, normally the transmittance of light that passes through a liquid crystal layer is controlled by a tilt angle of liquid crystal molecules that changes depending on a voltage that is applied between a pair of substrates (to the liquid crystal layer). Consequently, in such liquid crystal display devices, the transmittance depends on a viewing angle. As a result, in conventional liquid crystal display devices, depending on the viewing (observation) angle, display defects sometimes occur such as a decrease in the contrast ratio or gray scale inversion when a gradation display is performed. Accordingly, such conventional liquid crystal display devices generally have room for improvement with respect to viewing angle characteristics.
Domain division technology has been developed in which each pixel is divided into two or more regions in which the tilt directions of liquid crystal molecules are different. According to this technology, when a voltage is applied to a liquid crystal layer, because liquid crystal molecules tilt in different directions inside the pixels, the viewing angle characteristics can be improved. In this connection, the respective regions in which the alignment directions of the liquid crystal molecules are different are also referred to as “domains”, and domain division is also referred to as “multi-domain”.
Examples of liquid crystal modes in which domain division is performed include, with respect to horizontal alignment modes, multi-domain twisted nematic (TN) mode, multi-domain electrically controlled birefringence (ECB) mode, and multi-domain optically compensated birefringence (OCB) mode. In addition, examples of vertical alignment modes in which domain division is performed include multi-domain vertical alignment (MVA) mode, patterned vertical alignment (PVA) mode, and multi-domain VAECB (vertical alignment ECB) mode. Various modifications are being developed to realize even wider viewing angles with respect to the liquid crystal display devices of each mode.
Examples of methods of performing domain division include a rubbing method and a photoalignment method. As a rubbing method, a method has been proposed that performs a rubbing treatment with respect to an alignment film in a state in which a rubbing region and a non-rubbing region are separated from each other by a resist in which a pattern has been formed. However, in the rubbing method, an aligning treatment is performed by rubbing a vertical alignment surface with a cloth that is wound on a roller. Therefore, when the rubbing method is employed, foreign matter such as fiber of the cloth and scraped off pieces are generated, and defects such as damage, a characteristic shift, or deterioration in switching elements may arise due to static electricity. Hence, there is further room for improvement with respect to the rubbing method.
On the other hand, in the photoalignment method, a photoalignment film is used as an alignment film, and by irradiating (exposing) the photoalignment film with light such as ultraviolet light, an alignment regulating force is generated in the alignment film and/or an alignment regulating direction of the alignment film is changed. Thus, in the photoalignment method, aligning treatment of an alignment film can be performed in a contactless manner, and generation of dirt or foreign matter and the like during the aligning treatment can be suppressed. In addition, by using a photomask at the time of exposure, it is possible to irradiate light under different conditions in desired regions on the face of the alignment film. It is thus possible to easily form domains that have a desired design.
As examples of a conventional method of domain division using the photoalignment method, the following methods may be mentioned for a case where, for example, pixels are divided into two domains. More specifically, a method may be mentioned in which a photomask is prepared in which slit-shaped transparent portions that have a width that is equal to approximately half of a pixel pitch are formed in a light-shielding region, a first exposure is performed with respect to a half of the entire region of the pixel, and thereafter the photomask is shifted by an amount corresponding to approximately half of the pitch and a second exposure is performed under different conditions to the first exposure with respect to the remaining region of the pixel. According to such a method, each pixel can be easily divided into two or more domains. Furthermore, for example, Patent Literature 1 discloses technology that performs aligning treatment by a photoalignment method to forma VAECB (vertical alignment ECB) mode.
Further, the sizes of liquid crystal display device have been increasing in recent years. Manufacture and sales of liquid crystal television models has been increasing rapidly with respect to markets for model sizes in which plasma televisions have conventionally accounted for the major share of the market, such as the market for model sizes from 40 to 60 inches. However, it has been extremely difficult to perform domain division by a conventional photoalignment method such as the method described above with respect to a large liquid crystal display device in the 60 inch class. This is because an exposure apparatus that can complete exposure of a substrate in the 60 inch class through a single exposure and that is of a size that can be installed inside a factory does not actually exist at the present time, and hence it is not possible to expose the entire surface of a substrate in the 60 inch class through a single exposure. Consequently, when subjecting a large liquid crystal display device to domain division by a photoalignment method, it has been necessary to perform exposure several times to expose the entire surface of the substrate. In addition, even when a relatively small liquid crystal display device in the 20 inch class is subjected to domain division by a photoalignment method, a case can be considered in which it is required to complete exposure of the substrate by performing exposure several times when it is desired to keep the size of the exposure apparatus as small as possible. However, in a liquid crystal display device that has been subjected to domain division by exposing a substrate several times in the aforementioned manner, there are cases where a joint line between respective exposure regions can be clearly seen on the display screen, resulting in a defective product. Therefore, in the case of performing domain division of a liquid crystal display device by dividing the substrate into a plurality of regions and exposing the substrate through several exposures, there is still room for improvement with respect to improving the display quality and increasing the yield.
The inventors have developed the following method as technology for improving the above problems, and have already filed a patent application for this technology (see Patent Literature 2). That is, the method includes an exposure step of dividing the substrate face into two or more exposure regions and performing exposure of an alignment film through a photomask for each exposure region, wherein, in the exposure step, exposure is performed so that a part of adjacent exposure regions overlap with each other, and the photomask has a halftone portion that corresponds to the overlapping exposure regions.